1. Field of the Invention
This invention relates to a method of controlling a creep preventing device for a vehicle equipped with an automatic transmission. More particularly, the present invention relates to a method of controlling a creep preventing device which method can release the device from the operating state without any shock.
2. Description of the Prior Art
When a gear shift lever is kept set to a driving position (advancing position) during stoppage of a vehicle equipped with an automatic transmission, so-called "creep", in which the vehicle moves forward against the will of a driver, occurs due to the dragging torque of a torque convertor. To cope with this creep, the driver must keep pressing a brake pedal considerably strongly. This phenomenon is not advantageous for an engine, either, because idle opening of a throttle valve must be expanded excessively so as to offset this dragging torque and hence, the fuel consumption increases as much.
As one of the devices for preventing the creep, a device has been known in the past in which an oil discharge passage communicating with an oil tank is branched from an oil passage which supplies an operating oil pressure to a starting clutch of the automatic transmission, and an electromagnetic valve which closes the oil discharge passage under the demagnetized state and opens it under the excited state is disposed in the discharge oil passage. This device excites and opens the electromagnetic valve only during stoppage of the vehicle so as to escape the oil pressure supplied to the starting clutch to the discharge oil passage and to cut off the starting clutch and thus to prevent the creep of the vehicle. However, the following problems must be solved when putting this device into practical application.
To start the vehicle, the creep preventing device must be made to be inoperative and the starting clutch must be again connected. In this case, a certain kind of shock is likely to develop inevitably. A valve for progressively increasing a pressure or an accumulator may be employed, in principle, in order to establish re-connection of the starting clutch, but the use of such a valve or accumulator will result in the increase of size and complication of construction. Moreover, the oil passages will become more complicated in construction, and the increase in cost of production is unavoidable.